Azobenzene-containing Poly L-glutamic acid

the modified polymers were found to contain considerable amounts of N-acy-lureic groups originating from side reactions between DCCI and the y-carboxylic acid functions. The formation of N-acylureic groups was completely suppressed when the modification reactions were carried out in the presence of one equivalent of hydroxy-benzotriazole.[21,22] 

The maximum degree of photoconversion to the cis isomer (85%) is achieved by irradiating at 350-370 nm, whereas the maximum yield of the back-reaction from the cis to the trans isomer (80%) is achieved by irradiating at 450 nm. Using a 200 W lamp, irradiation for 1 or 2 minutes is enough to achieve the photostationary state. The thermal decay in the dark is much slower. At room temperature, it takes more than 200 hours to restore the fully trans isomeric composition. The photochromic cycles are completely reversible and can be repeated at will, without any apparent fatigue 21,22  

The conformational behavior can be quite different in aqueous solution. Below pH 5, a sample of poly(L-glutamic acid) containing about 30 mol% azobenzene units adopts a P-structure that is not affected by light. Above pH 7, the polypeptide is random coil and the conformation is, once more, not affected by the photoisomerization of the azo side chains. However, at pH values in the range 5-7 (close to the pK of the conformational transition), irradiation causes a remarkable diminishing of the ordered structure, which is completely reversed in the dark.120,221 

The effect was explained on the basis that the change in geometry of the azo moieties, caused by their trans-ds photoisomerization, might produce an increase in the local charge density of the environment around the helical backbone, thus destabilizing the ordered structure. 25 Unfortunately, the photoinduced conformational change was not reversible and the original structure could be not restored by irradiation at X 390 nm or upon dark-adaptation. 

The mechanism of the photoresponse was tentatively explained as follows. When azo units are in the planar, apolar, trans configuration, they merge into the hydrophobic core of the micelles, forcing the polypeptide chains to assume a coil conformation. Isomerization of the azo units to the skewed, polar, ds configuration inhibits hydrophobic interactions and causes the azo units to retreat from of the micelles, thus allowing the polypeptide chains to adopt the a-helix structure favored in the absence of micelles. In other words, the primary photochemical event is the trans-ds isomerization of the azobenzene 

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Ciardelli, F., Pieroni, O., and Fissi, A. (1986) Photocontrol of the solubility of azobenzene-containing poly(L-glutamic acid), J. Chem. Soc., Chem. Comm., 264-265. 

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